Rise of the Rail Splitter

Lincoln took his seat in the U.S. House of Representatives in December 1847. Hoping to distinguish himself in Congress, Lincoln chose to make opposition to the Mexican War one of his signature issues. He believed that the war was unnecessary and unconstitutional. In this letter, written back home to his law partner Herndon, a Whig who supported the war, Lincoln asserts that Polk violated the Constitution by invading Mexico not to repel an invasion but to seek additional territory. “Allow the president to invade a neighboring nation, whenever he shall deem it necessary to repel an invasion,” Lincoln wrote, “and you allow him to do so, whenever he may choose to say he deems it necessary for such purpose—and you allow him to make war at pleasure….If, to-day, he should choose to say he thinks it necessary to invade Canada, to prevent the British from invading us, how could you stop him?”